The use of home water treatment systems to treat tap water continues to grow dramatically in the U.S. and abroad, in part because of heightened public awareness of the health concerns associated with the consumption of untreated tap water.
Several different methods are known for filtration of water, and various devices and apparatus have been designed and are commercially available. These methods and devices vary depending on whether the application is for industrial use or for household use.
Water treatment for household use is typically directed to providing safer drinking water. The methods and devices typically used in households for water treatment can be classified into two basic types. One type is a pressurized system, such as a faucet mount system, and typically uses a porous carbon block as part of the filtration system. The other type is a low pressure system, such as a pour-through pitcher system, and typically uses activated carbon granules as part of the filtration system.
Filtration of water in a pressurized system has the advantage of the pressure to drive the filtration through the carbon block and therefore does not usually face problems of achieving desired flow rate while maintaining effective filtration of contaminants. However, when carbon blocks designed for pressurized systems are applied to gravity fed systems, they fail to produce the desired flow rates consistently over time.
Filtration of water in a low pressure system faces the challenge of undesirable contaminants while maintaining a desired high flow rate. However, when carbon blocks designed for pressurized systems are applied to gravity flow systems, they fail to produce the desired flow rates consistently over time.
Gravity flow filtration systems are well known in the art. Such systems include pour-through carafes, water coolers and refrigerator water tanks, which have been developed by The Clorox Company (BRITA®), Culligan™, Rubbermaid™ and Glacier Pure™.
Typically, these systems are filled with tap water from municipal supplies or rural wells, as the user wishes to remove chlorine and/or lead or other contaminants, or to generally improve the taste and odor of the water. These devices continue to be very popular, especially in view of the emphasis on healthy drinking water and in view of the expense and inconvenience of purchasing bottled water.
Prior Filter Blocks
Filter blocks for water filtration comprising granular activated carbon (GAC) and binder, with or without various additives such as lead sorbent, have been commercially available for many years. The raw materials are extruded or compressed into molds to form hollow, cylindrical or “tubular” blocks. Examples of conventional carbon blocks are given in Heskett U.S. Pat. No. 3,538,020, Degen U.S. Pat. Nos. 4,664,683 and 4,665,050, “Amway” U.S. Pat. No. 4,753,728, and Koslow U.S. Pat. Nos. 5,019,311 and 5,147,722 and 5,189,092.
The fluid-flow path through these hollow, cylindrical activated carbon blocks is generally radial. In out-side-in flow schemes, housing structure and internals distribute water to the outer cylindrical surface of the block, and the water flows radially through the inner, cylindrical wall to the hollow axial space at the center axis of the block. From the hollow axial space or perforated tube therein, the filtered water flows out of the filter at either at the bottom end or the top end of the filter, depending upon how the internals and ports have been designed.
These tubular filters have a single outside diameter “OD” (the outer cylindrical wall) and a single inside diameter “ID” (the inner cylindrical wall), with the two diameters defining a wall thickness. The cylindrical volume, minus the hollow space volume, defines the volume of filtering media. These tubular shapes have end surfaces opposing each other axially. These end surfaces are typically sealed to end caps to cause fluid to flow in a radial direction rather than around the end surfaces of the block. The ID, OD, and axial length define the surface areas, volume, and mass of the tubular-shaped activated carbon block. Activated carbon blocks can be varied in outside diameter, inside diameter, and length in order to achieve a specified volume and surface area of media.
The materials used to make radial-flow activated carbon blocks, as shown in the above-referenced patents and as discussed above, are typically carbon particles ranging from 12×30 US mesh to 80×325 US mesh (Koslow states 0.1 to 3,000 micrometers) and thermoplastic or thermo-set binders that are common to the art and disclosed in the referenced patents. Other materials can be blended with the carbon particles and binder particles such as lead-reducing sorbents.
Particle size, wall thickness, surface area, and compression can all be adjusted separately to achieve a desired pressure drop through a filter. Use of smaller carbon particles, increased compression, or thicker walls will generally increase pressure drop and increase contaminant removal. Use of larger carbon particles, less compression, or thinner walls will generally decrease pressure drop and decrease contaminant removal. Larger diameters (OD and ID) for cylindrical blocks will decrease pressure drop by increasing surface area available to the fluid. A large OD carbon block with a small ID will have more pressure drop than the same carbon block with a larger inside ID, as the length of the fluid path through the block is longer.
Corrugated Filter Sheets for Air Filtration
Clapham, in U.S. Pat. No. 3,721,072, produces a low-pressure air filter by providing a monolithic extended surface filter sheet, in the form of a wave pattern. Each wave of the extended surface consists of a peak and a trough extending along the entire length of the filter body to the outside boundary of the filter. ('072 FIG. 1). Clapham's wave forms are much smaller than the overall dimensions of the filter body, for example, thirteen waves in a single filter body, and the filter body is substantially wider and longer than it is thick, for example, typically more than 10 times as long (or at least more than 5 times as long) and also more than 10 times as wide (or at least more than 5 times as wide) as the thickness of the filter body. Therefore, the filter body may be considered a corrugated filter sheet or plate. Clapham's sheet-like or plate-like filter body may be placed in a frame, extending around the periphery of the filter body, made of “metal, glass, wood, plastic, paperboard, and the like . . . or bonded carbon integral to the filter.”
Chapman, in U.S. Pat. Nos. 6,322,615 and 6,056,809, discloses corrugated sheets for air filtration, wherein, as in Clapham, the peaks and troughs extend all the way to the outside boundary of the filter, the wave forms are much smaller than the overall dimensions of the filter body, and the filter body may be considered a corrugated sheet or plate. Methods of making this corrugated filter body comprise rolling the filter material between rollers with multiple V-shaped tools forming the peaks and troughs in the extended surface of the filter body.
Gelderland, et al., in U.S. Pat. No. 6,413,303, disclose activated carbon air filters made of layers of corrugated paper sheets coated in carbon and binder. Insley, et al., in U.S. Pat. No. 6,280,824, disclose polymeric film layers comprising filtration media and each having a corrugated shape. Gelderland, et al. and Insley, et al. each teach flow being through the open spaces defined by the corrugates (parallel to the corrugate troughs), rather than through the corrugated plates (i.e. parallel, rather than transverse, to the plane of each plate).
Granular Activated Carbon Media for Water Filtration
Granular activated carbon (GAC), without binder and with or without various additives such as lead sorbent, has been used in water filtration for years. The granular activated carbon is typically loaded into a compartment inside a filter housing to act as a filter or carbon “bed.” The housing and internals are adapted to contain the otherwise-loose granules in place in the compartment, and to distribute water to the inlet of the bed and collect the water at the outlet of the bed. A bed of GAC, with optional other granular media or additives, is the media of choice for low pressure or gravity flow applications, because of the relatively low pressure drop through the bed of granules; no binder is present and, hence, no binder fills the spaces between the carbon granules to interfere with the flow. The interstitial spaces between the granules allow water flow through the bed with good media contact but without the pressure drop that might be expected in a compressed, binder-formed block.
These gravity-flow filtration devices typically feature relatively small, disposable and replaceable filter cartridges that are inserted into the device and used for several weeks of normal use. Examples of these devices and/or of filters that are designed for these devices are disclosed in U.S. Design Pat. No. 416,163, U.S. Design Pat. No. 398,184, U.S. Pat. No. 5,873,995, U.S. Pat. No. 6,638,426, and U.S. Pat. No. 6,290,646. The filters for these devices are entirely or substantially comprised of beds of granular media.
The filtration cartridge typically employed in pour-through (or gravity flow) systems hold blended media of approximately 20×50 mesh granular activated carbon and either an ion exchange resin, which most typically contains a weak acid cation exchange resin, or a natural or artificial zeolite that facilitates the removal of certain heavy metals, such as lead and copper. Weak acid cation exchange resins can reduce the hardness of the water slightly, and some disadvantages are also associated with their use: first, they require a long contact time to work properly, which limits the flow rate to about one-third liter per minute; second, they take up a large amount of space inside the filter (65% of the total volume) and thus limit the space available for activated carbon.
A further problem associated with blended media of granular carbon and ion exchange resin is that they have limited contaminant removal capability due to particle size and packing geometry of the granules. When large granules are packed together, large voids can form between the granules. As water passes through the packed filter bed, it flows through the voids. Much of the water in the voids does not come into direct contact with a granule surface where contaminants can be adsorbed. Contaminant molecules must diffuse through the water in the voids to granule surfaces in order to be removed from the water. Thus, the larger the voids, the larger the contaminant diffusion distances. In order to allow contaminants to diffuse over relatively long distances, long contact time is required for large granular media to remove a significant amount of contaminant molecules from the water.
Conversely, small granules (i.e., 100-150 μm) form small voids when packed together, and contaminants in water within the voids have small distances over which to diffuse in order to be adsorbed on a granule surface. As a result, shorter contact time between the water and the filter media is required to remove the same amount of contaminant molecules from the water for filter media with small granules than for filter media with large granules.
But there are some drawbacks to using filter media with small granules. Water flow can be slow because the packing of the granules can be very dense, resulting in long filtration times. Also, small granules can be more difficult to retain within the filter cartridge housing.
Good flow distribution in the filter is of primary concern in low pressure or gravity flow systems such as in water pitcher devices, water cooler devices, and other systems mentioned above, because flow distribution affects filtration effectiveness and the time at which “breakthrough” of contaminates occurs, and, hence, the time at which the filter should be changed out. As these filtration systems typically do not contain any means for monitoring filtration effectiveness or breakthrough, and, at most, have means for measuring total water that has passed through the filter, it is important that good flow distribution be maintained to maximize use of media, and, hence, to maximize the filtration effectiveness for a given volume of filtered water. If channeling occurs at any time during the filter life, the effectiveness of the filter and/or the effective filter capacity is reduced, and the filtered water quality may drop if the filter is not changed out.
Good water flow rate through the filter is also of primary concern in low pressure or gravity flow water systems such as a water pitcher device, water cooler device, or the like because this affects how quickly filtered water from a freshly-water-filled device may be used. Typically, these devices are kept in the refrigerator or on a countertop, and so their total volume is kept at an amount that is reasonable for such spaces and that is of a reasonable weight to carry. Users of such devices typically do not want to wait a long time for the filtered water. Therefore, reasonable flow rate through the filter is important for customer satisfaction and to gain a competitive edge in the marketplace. As these water filtration devices typically utilize only gravity to force the water through the filters, achieving adequate flowrate of water through the filter is problematic, especially in view of the goal of effective contaminant removal and long filter life. The goal of low pressure drop for high flowrates would drive the design toward short granular filter beds, but the goal of effective contaminant removal and long life without breakthrough would drive the design to in the opposite direction, toward long filter beds. Further, achieving adequate flowrate is also problematic because the carbon-based granular media that are used in the filters in question tend to be slightly hydrophobic. Therefore, while excellent water-media contact is needed for good flow distribution and good flow rates, the media actually tends to resist wetting by the water it is intended to filter.
Therefore, conventional filters for water pitcher devices have typically included GAC beds about 2-6 inches deep. Further, important procedures in the installation of a filter into one of these water pitcher devices are the pre-rinse and the pre-wet steps recommended by manufacturers of the devices and the filters. These procedures involve rinsing the filter and then soaking the fresh filter in water for several minutes prior to inserting the filter into the device. These procedures are explained by the manufacturers as steps that remove carbon fines that may reside in the fresh filter, and that wet the granular or particulate carbon media to achieve better flow distribution and flow rates after the filter is installed in the device.
It is believed that there is room for improvement in the filters used for gravity flow water filtration devices, such as water pitchers, carafes, countertop tanks, and water coolers and other filters that are used for low pressure systems (such as 30 psi or less).
It would be useful to have gravity flow filters that exhibit both good water flow rates and high contaminant reduction.